Bass guitar beyond a certain level is very difficult and I will justify that. Bass at a basic level is a relatively easy instrument - no drama there. But to break 'through the wall' and actually be a 'good' bass player is remarkably difficult. There are far fewer really, really good bass players (per ratio of players) than there are guitarists and I think there is a reason. It's very difficult to learn expression on an instrument that has its sole focus on the rhythm section of a band and playing the bass alone is not an inspiring experience, unlike the 'Cello or Violin. In order to really appreciate the bass guitar, you have to practice for hours and hours, getting your feel right and learning to interact with other musicians and as a result, practicing the bass is a very hard thing to do for long periods.

Bass is inherently a simple instrument, but it requires so much more dedication to practice.